Body joints normally have two functions, the primary being to allow mobility while the secondary function is to transmit force.
The primary function enables, for example, a distal bone, e.g. some part of a distal appendage or digit, to assume a variety of positions vis-a-vis the proximal bone, digit or body member.
In the transmission of force, the joint acts mechanically to transfer forces from one bone to the other through the articulation, usually in directions which are perpendicular to the articulation surfaces. Thus the articulation or joint must be capable of force transfer in the manner described while allowing control of the relative movement of the bones.
However, it is important to note that the articulation, especially for certain body members, must also be capable of taking up or resisting support forces which may be perpendicular to the articulation axis, transverse to the articulation surfaces and even perpendicular to the axis of an artificial joint. Such support forces are those which are applied, for example, against the palm of the hand or the sole of a foot.
In conventional prosthetic devices for replacing the natural joints, the body has a flexible central region forming the articulation axis and extensions therefrom which are anchored in the respective bones. This central region has been found to have limited stability and transverse strength, i.e. to be insufficiently able to handle the longitudinal and transverse forces which may be applied to the prosthesis.
Even when the prosthetic device is capable of transmitting longitudinal force without difficulty, problems are nevertheless encountered when force is applied transverse to the longitudinal direction as is the case when pressure is applied to the palm of the hand or the sole of the foot. Frequently, moreover, an abnormal stress is applied to the articulation as when a finger or toe is bent backward. Here, too, conventional joint replacement implants have proved to be ineffective.
When abnormal stresses or abnormally directed stresses are applied to the flexible and elastic body, the deformation of the latter appears to be localized particularly at the junction between the extension and the bending zone, thereby tending to rupture the implant. The concentration of pressure which is also a characteristic of earlier systems, at locally weak or sensitive areas, can cause failure of the material with deterioration or instability of the prosthetic device and even discomfort.